This invention relates to a lightweight bead for radial ply tires, more particularly a heavy-duty lightweight bead for tires subjected to very heavy loads and high pressures.
Annular tensile members, commonly referred to as tire bead cores, are designed to securely hold the tire on the rim during use.
A tire rim generally has a rim flange and a bead seat specifically designed to hold the bead portion of the tire in place.
These rims are designed to specific dimensions and tolerances as prescribed by various industry associations. In the United States the Tire and Rim Association sets all rim standards. In Europe, the European Tire and Rim Association sets the rim standards. In Japan the J.P.T.O. sets these standards. For the most part, these rim standards globally insure proper tire fits can be reliably designed. This standardization enables tire makers to design tire beads that safely can be mounted and retained on the rims.
The tire beads provide a radially inner portion between the bead core and the rim that is radially compressed and as this portion is compressed the bead core is placed in tension. Radial compression occurs as the tire is mounted on a tapered rim seat by the action of the internal pressure of the tire pushing the bead axially outwardly toward the vertical bead flange. The tapper on rims typically are about 5xc2x0 or in the case of commercial truck tires as much as 15xc2x0.
What actually makes the bead retaining forces achieve very high values is the bead core. Typically the bead cores are made of one or more steel wires wound in an annular configuration to form a bead bundle in the shape of a circular hoop of any variety of cross-sections. Some bead cores are circular in cross-section, others are square, rectangular, hexagonal or variations of those shapes.
In the past, attempts have been made to make synthetic beads or non-ferrous type bead cores. These bead cores have been limited to use in toys or bicycles. The use of synthetic bead cores achieve a light weight structure but generally at the expense of lower tensile strength or bead wire fretting.
Recently in several Japanese patent publications, the use of a combination of steel wire and aramid cores has been proposed as a lightweight bead core. Yokohma Rubber Company, LTD, in JP4078703, suggests using an arimid fiber drawn in a non-twisted state as a core member which is wound by a steel wire in a spirally wound state on the outside of the arimid core. The result is a circular bead core having a lighter weight than an all steel bead core of similar cross section.
Similarly Sumitomo Rubber Ind., LTD in JP4183614 and Toyo Tire in JP7096720 also use a combination of aramid fiber and steel wire to make a bead core. The Toyo design requires at least a first layer on the core inside diameter being of steel wire. The subsequent layers can be made of aramid fiber cords. This insures the bead does not experience a lowering of lifting force when compared to an all-synthetic bead core. In the Suminto concept alternating layers of Aramid cords and steel wires are used. In one configuration, the steel wire and aramid cords are arranged in vertical or radial layers and in another embodiment the aramid and steel are layered horizontally to make a bead core of square or rectangular cross-section.
In each case, care is taken to insure the synthetic fiber is used in a non-twisted configuration. The primary issue with synthetic cords is that when provided in a cable that has the cords twisted there is created several problems. The first is called creep under load. The synthetic cables or cords will stretch under load and as the plastic flows the restraining force actually will lower with time, accordingly the use of steel in a radially innermost layer is essential if bead retention forces are to be reliably constant. A second problem with such beads is a pheoromina called fretting, Aramid, in particular, and many other plastics have a condition where small brittle fractures occur if the cords are placed in compression. Cabling such cords actually increase the likelihood of creating these fractures. Accordingly, aramid cords are not used in the carcass plies and are generally not used in belt structures because repeated exposure to compression stresses creates cord breakage. In a bead core, almost all of the loads are in tension except when the bead is helically or spirally wound. In those cases the cords work against each other creating small bending forces, which over time result in minute abrading friction of the adjacent cords. This gives rise to a phenomenal referred to as fretting.
The lightweight bead shows promise in small, lightly loaded tires for passenger vehicles but heretofore have not been considered practical in very heavy-duty load conditions.
In radial airplane tires where lightweight issues are very important. The use of steel bead cores has been the practice. The tires are inflated to about 200 psi (14.1 kg/cm2), and can experience impact loads of 50,000 lbf (22,680 kg) or greater.
In such tires, tests are conducted to insure adequate safely margins exist. Typically the tires annular tensile members are designed to exceed the strength of the tire""s carcass and belt structure. Hydraulic burst test are conducted to failure where water is injected into the tire until the tire fails at some very high pressure. Typically the beads survive these tests with the failure mode occurring in the belts or the plies.
In large off-road tires used in earthmoving equipment. The tire""s beads are constructed of steel wire formed in large bundles. The tires operated at very high pressures, typically 100 psi and the bead bundles may be greater than an inch (2.54 cm) across and comprised of hundreds of steel wires.
Similarly commercial truck tires use all steel bead cores. These tires run at about 95 psi or greater and must carry very large loads. hi each case the use of all steel bead cores has been the accepted practice.
While lightweight tires are generally understood to run cooler, it has been generally understood that those benefits are derived from a reduction of carcass rubber or tread rubber and not by a reduction in the weight of the bead core.
For these reasons the interest in lightweight bead cores for heavy duty tires has received little attention. Only in aircraft tires has the tire weight issue been considered sufficiently important.
The present invention is directed to a lightweight bead core for heavy-duty tires. The invention was first formulated for an advanced lightweight radial aircraft tire. The analysis has shown that the concept is so cost effective and durable that it can be used in almost any heavy-duty tire application, including those mentioned above as well as for farm tires.
The object of the invention was to provide a lightweight, yet high strength bead core for severe service applications such as aircraft tires.
A further object of the invention was to provide a low cost bead core that was competitively priced relative to the all steel bead cores.
These features as well as others are described in the invention described hereinafter.
A pneumatic radial tire having a bead portion provided therein has a bead core formed by a plurality of sheath wires enveloping a central core.
The sheath wires are steel and the central core is made from a lightweight alloy material having a weight less than steel. The central core material is selected from the group of titanium, aluminum, magnesium, or other metal alloy.
In one embodiment, the bead core has the sheath wires helically wrapped around the central core. The bead core can be of circular, rectangular, square, or hexagonal cross-section, or a combination of such cross-sectional shapes.
The central core can be a single wire or rod wrapped 360xc2x0 or more. Alternatively, the central core may have a plurality of wires wrapped 360xc2x0 or more.
xe2x80x9cApexxe2x80x9d means a non-reinforced elastomer positioned radially above a bead core.
xe2x80x9cAspect ratioxe2x80x9d of the tire means the ratio of its section height (SH) to its section width (SW) multiplied by 100% for expression as percentage.
xe2x80x9cAxialxe2x80x9d and xe2x80x9caxiallyxe2x80x9d means lines or directions that are parallel to the axis of rotation of the tire.
xe2x80x9cBeadxe2x80x9d means that part of the tire comprising an annular tensile member wrapped by, or otherwise attached to ply cords and shaped, with or without other reinforcement elements such as flippers, chippers, apexes, toe guards and chafers, to fit the design rim.
xe2x80x9cBelt or breaker reinforcing structurexe2x80x9d means at least two layers of plies of parallel cords, woven or unwoven, underlying the tread, unanchored to the bead, and having both left and right cord angles in the range from 17xc2x0 to 33xc2x0 with respect to the equatorial plane of the tire.
xe2x80x9cBias ply tirexe2x80x9d means a tire having a carcass with reinforcing cords in the carcass ply extending diagonally across the tire from bead core to bead core at about a 25xc2x0-50xc2x0 angle with respect to the equatorial plane of the tire. Cords run at opposite angles in alternate layers.
xe2x80x9cCarcassxe2x80x9d means the tire structure apart from the belt structure tread, under tread, and sidewall rubber over the plies, but including the beads.
xe2x80x9cCircumferentialxe2x80x9d means lines or direction extending along the perimeter of the surface of the annular tread perpendicular to the axial direction.
xe2x80x9cChafersxe2x80x9d refers to narrow strips of material placed around the outside of the bead to protect cord plies from the rim, distribute flexing above the rim, and to seal the tire.
xe2x80x9cChippersxe2x80x9d means a reinforcement structure located in the bead portion of the tire.
xe2x80x9cCordxe2x80x9d means one of the reinforcement strands of which the plies in the tire are comprised.
xe2x80x9cEquatorial plane (EP)xe2x80x9d means the plane perpendicular to the tire""s axis of rotation and passing through the center of its tread.
xe2x80x9cFlipperxe2x80x9d means a reinforced fabric wrapped about the bead core.
xe2x80x9cFootprintxe2x80x9d means the contact patch are area of the tire tread with a flat surface at zero speed and under normal load and pressure.
xe2x80x9cInnerlinerxe2x80x9d means the layer or layers of elastomer or other material that form the inside surface of a tubeless tire and that contain the inflating fluid within the tire.
xe2x80x9cNet-to-gross ratioxe2x80x9d means the ratio of the tire tread rubber that makes contact with the road surface while in the footprint, divided by the area of the tread in the footprint including non-contacting portions such as grooves.
xe2x80x9cNormal inflation pressurexe2x80x9d refers to the specific design inflation pressure and load assigned by the appropriate standards organization for the service condition for the tire.